Adaptive Cruise (i.e., speed) Control (ACC) systems operate much like conventional Cruise Control systems, with the added capability of being able to sense in-path vehicles and to slow the ACC equipped vehicle in response. An ACC equipped vehicle thereby allows its operator to automatically control the vehicle speed, as with conventional Cruise Control, without the necessity of having to deactivate and reactivate control whenever slower traffic is encountered.
As is well known in the art, existing ACC methods and systems use a forward looking range sensor such as radar to sense an in-path vehicle (which may also be referred to as a sensed target or primary target). Based on the radar sensor information, such ACC methods and systems then determine the range and relative velocity (or range rate) of the sensed in-path vehicle. Using the range and range rate, the speed of the ACC equipped vehicle is controlled to maintain a selected following interval between the ACC equipped vehicle and the sensed in-path vehicle. The speed of the ACC equipped vehicle is typically controlled by automatic control of the vehicle throttle actuator. In more advanced ACC methods and systems, vehicle speed may also be controlled by automatic control of vehicle brake actuators. Such ACC methods and systems have the ability to apply a moderate degree of braking to the vehicle to achieve further vehicle deceleration (i.e., in addition to vehicle deceleration achieved via throttle control) in response to an in-path vehicle.
When maintaining the selected following interval, however, existing ACC methods and systems may decelerate the ACC equipped vehicle to a speed less than a minimum operating speed threshold. In such situations, the ACC system is simply deactivated. This is typically accomplished by "fading" the vehicle brakes. That is, the vehicle brake actuators are controlled to gradually decrease to zero the hydraulic pressure applied to the vehicle brakes. Thereafter, the ACC system having been deactivated, absent intervention by the vehicle operator, the vehicle simply coasts.
Thus, in an ACC system, there exists a need for a method and system for continued control of an ACC equipped vehicle when the speed of the vehicle decreases below the minimum operating speed threshold and a sensed target disappears. Such a method and system would add to an ACC equipped vehicle the ability to control the vehicle speed based on a phantom target. More specifically, when maintaining the selected following interval, if the speed of the ACC equipped vehicle decreases below the minimum operating speed threshold, such a method and system would determine whether the sensed in-path target vehicle has disappeared (such as where the sensed target leaves the path of the ACC equipped vehicle, the ACC equipped vehicle leaves the path of the sensed target, or the sensed target reaches a sufficiently low speed that it can no longer be identified). In that event, such a method and system would compensate for the loss of the sensed target by controlling the speed of the ACC equipped vehicle based on the range and range rate of a phantom target. Such a phantom target would be created from the last known range and range rate of the sensed target that has disappeared from the radar tracking files of the ACC system. Below the minimum operating speed threshold, such a method and system could thereby allow, for example, the ACC equipped vehicle to maintain braking even if the in-path sensed vehicle has disappeared. In so doing, such a method and system would provide the ACC equipped vehicle with a smoother, more comfortable response.